Explosion characteristics and critical conditions triggered by the self-ignition of deposited pulverized coal

In pulverizing systems, elevated ambient temperatures make coal dust deposited on hot surfaces prone to spontaneous ignition; once entrained by airflow into the surrounding space, such deposits may further evolve into coal dust cloud explosions, posing major risks to safe operations. The objective of this study is to reveal the critical conditions and kinetic characteristics under which self-ignition of deposited coal dust initiates explosions in such systems.

In this study, a coal dust combustion—explosion experimental platform was combined with Fluent numerical simulations to conduct a systematic investigation of the self-ignition, ejection, and explosion processes of deposited coal dust under heated environments, as well as behavioral characteristics of these processes. First, a coal dust combustion—explosion experimental system capable of precise control of ambient temperature and hot-plate temperature was established; on this basis, the internal temperature distribution of coal dust layers at different hot-surface temperatures and the critical conditions for self-ignition were examined. Second, ejection tests were conducted with dust-layer center temperatures of 260-380 ℃ to analyze the formation, ignition, and explosion of deposited coal dust clouds. Finally, based on the experimental results, a Fluent-based numerical model was developed for the post-lofting processes, including moisture evaporation, devolatilization, gasphase combustion, and char combustion. The particle trajectories, velocities, and temperature evolution during lofting were analyzed, and the critical oxygen concentration and dust concentration required to induce an explosion were determined.

Experimental results show that the hotplate temperature required for thermal runaway decreases with increasing dust-layer thickness. For a 4 mm coal dust layer, the critical hotplate temperature for self-ignition is 255 ℃; when the thickness increases to 10 mm, this temperature drops to 225 ℃. When the internal temperature of a self-ignited dust layer lies within 275-395 ℃ (corresponding to a center temperature of 300-340 ℃), the resulting coal dust cloud can trigger an explosion. The lofting process can be divided into three stages—rapid ejection, decelerating diffusion, and free diffusion—with explosions occurring mainly in the latter two; the maximum particle velocity is approximately 60 m/s, and the peak temperature is approximately 2400 ℃. Mechanistically, volatiles released from coal particles undergo homogeneous combustion outside the particles, whereas char undergoes heterogeneous combustion within the particle interior; these simultaneous phenomena significantly elevate the flame-core temperature. As the ambient temperature increases, both the critical coal dust mass concentration and the critical oxygen concentration for explosion decrease: when the ambient temperature rises from 120 ℃ to 200 ℃, the critical dust cloud concentration decreases from 380 to 95 g/m³, and the critical oxygen concentration decreases from 21% to 13%.

Combustible gases (e.g., CO) generated during the self-ignition stage accumulate and are subsequently ignited by high-temperature particles, initiating gas-phase combustion. The resulting heat release then ignites suspended coal dust particles, triggering solid-phase combustion. This sequence constitutes a critical pathway by which self-ignition escalates into an explosion. These findings provide a theoretical basis for explosion prevention and control in pulverizing systems and offer practical guidance for risk mitigation measures, including hot-surface temperature control, dust-layer thickness management, ventilation and oxygen concentration limits, and operational strategies that minimize the lofting of preheated deposits.